A class of insecticide that is applied to seeds and taken up into plant tissue may be responsible for much of the widespread decline in honeybee populations, increasing numbers of researchers and environmentalists are suggesting.

Starting in 2005, beekeepers in the United States first reported large numbers of bees mysteriously disappearing, and since then the problem has spread to different parts of the world. No one cause of the collapse has been identified, although front-running theories include parasites, viruses, stress from long-distance transport of hives for pollination, and pesticides.

“We do feel like pesticides are playing a role in pollinator decline,” said researcher Maryann Frazier of Penn State University. “We know that the pesticides are there. We don’t know yet exactly what role they’re playing.”

The recent documentary “Nicotine Bees” makes the case that a new class of pesticides, introduced in 2005, may be the primary cause of what has come to be known as Colony Collapse Disorder. The chemicals, known as neonicotinoids, are synthetic form of nicotine that is applied to seeds and taken up by the plant into its tissue as it grows. Insects that eat any part of the plant are then killed. Because neonicotinoids were introduced worldwide at around the same time as the first cases of colony collapse, and because they have been directly linked in some cases to massive bee die offs, they have emerged as a primary suspect in the disorder. The California Department of Pesticide Regulation has launched an investigation into some neonicotinoids, and the Sierra Club has asked the Environmental Protection Agency (EPA) to ban them all until they can be proven safe.

“What we’re asking the EPA is to go with precautions,” said the group’s Laurel Hopwood. “Let’s go ahead and suspend them until we get all of the research completed.”

Neonicotinoid use has already been restricted in France, Germany, Italy and Slovenia.

“Some of us think we’ve got enough chemicals out there killing bees,” said beekeeper John Talbert. “Which begs the question: What is it doing to people?”